The geopolitical backdrop of Moon's visit is framed around an increasingly provocative regime in North Korea and an administration in the US that seems focused on forcibly changing the calculus of Pyongyang's decision to develop and maintain a nuclear weapons programme. The stakes will be especially high because Moon - a progressive with dovish tendencies towards Pyongyang - is looking to demonstrate unity with President Trump without sacrificing his own policy goals of engaging the North diplomatically. There are also valid concerns in Seoul on Trump's protectionist rhetoric and his desire to amend - or maybe even withdraw from - the US-Korea Free Trade Agreement.
During Moon's election campaign, his rivals - from both the conservative and progressive sides - launched salvos at him outlining his dubious approach to dealing with North Korea and his sceptical take on the US-South Korea alliance remaining at the core of Seoul's security ethos. They questioned Moon's flip-flopping on the deployment of the Terminal High Altitude Area Defense (THAAD) anti-missile system in South Korea and his pledge to "review" the decision.
Despite THAAD being already deployed and operational, Moon has now made good in some sense on that pledge through his suspension - pending a probe's findings - on the deployment of an additional four missile launchers (there are already two launchers active and operational).
Washington's position, before the Moon-Trump meeting, is that Seoul's change of heart on THAAD is not only upsetting the operational effectiveness of the missile defence system (which is aimed to both defend South Korea and the more than 30,000 US troops in the country) but also is simultaneously weakening the credibility of the deterrence value of the US-Korea alliance. Moreover, some reports have indicated that Trump himself is "furious" over Moon's decision to suspend the full deployment of the launchers.
Now that Moon's place in the Blue House is a reality, there must be a trained eye focused on Seoul's foreign policy orientation. Will Moon really look to implement a dovish and neo-Sunshine policy approach with regard to North Korea? How will the Moon administration look to repair strained ties with China and also Japan? And how will relations evolve between Moon Jae-In and Donald Trump, two leaders from the opposite ends of the political spectrum?
Initial indications should stoke scepticism on how Moon may affect the regional security landscape and the US-Korea alliance, but we should also be cautious not to paint his administration into a corner.
On China, Moon has extended an olive branch to Beijing by indicating his desire to "negotiate" on the issue of THAAD in concert with the US. This is a large change in tone from the former government of disgraced Park Geun-hye, which was irate at Chinese interference on the THAAD issue and rebuked Beijing for its insistence that the defence system should not be deployed.
Some in Beijing may celebrate Moon's place in power and think that they can now look to pry away South Korea from its strong relationship with the US. Indeed, China's primary countermove in the region has been to weaken US security leverage in the region by eroding the substance of its alliances. However, Moon's win is not a strategic victory for China. The more realistic analysis shows that Beijing got bailed out due to domestic political issues in Seoul surrounding the impeachment of Park Geun-hye.
There should be a great deal of caution on how Moon and Trump will interact and how the intersection of Washington's "Maximum Pressure and Engagement" dovetails with Moon's desire to retrofit Seoul's hardened approach to the North. But, in some sense, there could be common ground between the two - both have signalled engagement is a possibility, including even meeting with leadership in Pyongyang. Moreover, despite Washington's frustration with Moon's decision on THAAD - the Trump administration needs South Korea, in addition to other regional players like Japan and China, on board to follow through on its approach to Pyongyang.
Despite these structural forces reinforcing the US-Korea alliance, there are also glaring divergences potentially with strong headwinds in the coming months. Moon seems disenchanted with the "deterrence plus" approach at turning the screws on North Korea and will likely push back on the need for talks on more robust trilateral security cooperation with Japan. Moon is also likely to brush off suggestions of regional missile defence - and has previously bristled the Japan-South Korea GSOMIA intelligence-sharing pact. This is true even despite the positive news of a first-ever hotline established between ministers of defence in Seoul and Tokyo.
Initial indications should stoke scepticism on how Moon may affect the regional security landscape and the US-Korea alliance, but we should be also cautious not to paint his administration into a corner. First, it is important to recognise that - despite the fact that security and foreign policy issues played a large role in the domestic and international media coverage leading up this month's election - Moon was mainly elected on his economic policies and his seizure of the electoral space in the wake of Park Geun-hye's impeachment. Second, Moon is likely to be hemmed in - to some extent - on his new designs for foreign policy by the opposition parties in the National Assembly and also regional provocations from the North. These structural factors will hopefully push Moon to pragmatism in regional relations.
J Berkshire Miller is the director of the Council on International Policy and is a fellow on East Asia for the EastWest Institute.
The views expressed in this article are the author's own and do not necessarily reflect Al Jazeera's editorial policy.